The Sultan Ahmed Mosque is a historic mosque, popularly known as the Blue Mosque for the blue tiles adorning the walls of its interior.
It was built from 1609 to 1616, during the rule of Ahmed I. Its Külliye contains a tomb of the founder, a madrasah and a hospice. The Sultan Ahmed Mosque is still popularly used as a mosque: http://www.en.wikipedia.org/wiki/Sultan_Ahmed_Mosque